The conducted research includes, according to the specific objectives, (i) development of the multiresonance concept for subwavelength structures and quantification of the scaling of resonance frequencies, (ii) design and experimental study of ultra-miniature antennas with the combined effect of subwavelength resonators and a (very-)high-permittivity substrate; (iii) assessment of applicability of the ideas of Transformational Optics for design of ultra-miniature antennas, (iv) compact planar antennas based on the multiresonance concept, and (v) investigation of metasurfaces and related structures that (may) inspire multiband advanced antennas or sub-/superstrates for antennas, and metasurfaces inspired by the proposed antennas.
The multiresonance concept has been developed based on the guess that multiple subwavelength resonators may provide such a high field confinement that they work rather separately from each other in one antenna, or affect each other in a desirable and predictable way. Quantification of the scaling of resonant frequencies, which occurs at substrate’s permittivity variation and fixed size of the substrate-resonator block of antenna, has been introduced. The obtained results allow us to understand and exploit the specifics of the scaling in the open resonance structures and, consequently, justify a particular permittivity range needed to choose a proper substrate material when the maximal size of the antenna is strictly limited. Various natural materials have been assessed as substrates of ultra-miniature antennas for the range between 0.8 and 10 GHz. The choice has been made in favor of lithium niobate (LiNbO3), an affordable high-permittivity material (diagonal components of the relative permittivity tensor are 43, 43, and 28), which is widely used in electro-optics but not yet in ultra-miniature antennas. Fabrication steps are well developed for this material that makes it a perfect candidate for practical use. To validate the concept, an ultra-miniature dual-band monopole antenna on LiNbO3 substrate has been designed, fabricated, and studied experimentally. The antenna has an unusually small size: side size of its substrate-resonator block is just 1/24 of the wavelength at the lowest operating frequency (around 2.8 GHz). The obtained efficiency of 8-10% is acceptable for such small antennas. Good coincidence of the experimental and numerical results has been achieved. Some ideas of Transformational Optics approach were examined for possible use in design of ultra-miniature antennas.
The multiresonance concept has been adapted to the compact planar antennas on a conventional commercial substrate (Roger 5870, relative permittivity is 2.33). Several antennas having up to six bands were designed, whose unusual feature is that the subwavelength resonators are placed on the both sides of the substrate. This is crucial for compactness of the antenna since the maximal size is strictly limited. Almost arbitrary mutual location of the bands can be obtained for a given size of the substrate-resonator block. For the demonstration purposes, two antennas with three and four bands were fabricated and experimentally tested. The results are in good coincidence with simulations. The experiments demonstrate the usefulness of these antennas for sensing. All experimentally tested antennas were fed by a standard coaxial but are flexible regarding the feeding scheme.
Metasurfaces which (may) inspire advanced antennas and substrates and superstrates for radiated wavefront manipulation were studied. In turn, the designed ultra-minature/compact antennas were considered as unit cells of future metasurfaces. The transmission, reflection, absorption, and polarization conversion properties of the metasurfaces of the selected types have been studied in detail by means of numerical simulations. Connection between the properties of metasurfaces and their unit cells was demonstrated. Sample metasurfaces were designed for the purposes of future research.
